In general, a motor is classified into a direct-current electric motor or an alternating-current electric motor, depending on the kind of electric power applied thereto. In addition, such a motor is classified into an inner rotor-type motor or an outer rotor-type motor, depending on the arrangement of the stator and the rotor thereof.
An inner rotor-type motor has an arrangement in which the rotor is rotatably installed inside of the stator, and an outer rotor-type motor has an arrangement in which the rotor is rotatably installed outside of the stator.
Among the various types of motors described above, the outer rotor-type motor is employed in various fields as a direct-coupled motor for a washing machine, an in-wheel motor for an electric vehicle, or the like.
Among the above-mentioned outer rotor-type motors, the in-wheel motor can be improved in performance if the cooling efficiency thereof is increased. However, because such a conventional in-wheel motor is poor in cooling efficiency, it is difficult to maximize the performance thereof.
Especially, in order to improve the performance of an outer rotor-type motor, it is most effective to increase the cooling efficiency of the stator thereof. However, because such an outer rotor-type motor is adapted to cool the stator thereof by forming ventilation openings in the rotor thereof in the prior art, the cooling effect achieved thereby is insufficient for obtaining the substantial improvement of the performance of the motor.
Furthermore, because the ventilation holes are formed in an area where the rim wheel of a vehicle wheel is positioned, the air outside of the stator is very minutely introduced into the stator when the vehicle runs. Consequently, it is natural that the practical cooling efficiency is poor.